In modern displays, white LEDs are used to create the white light used to backlight the LCD. It is desirable to have the ability to vary the level of the backlight used. This is desired for both maximizing contrast as well as adjusting the display to the ambient light level. Conventional LED driver circuits accomplish dimming by adjusting the on time (duty cycle) of an LED string, such that the percentage of on time creates an equivalent brightness (or average intensity) at the desired brightness.
Some LED driver circuits include an integrated circuit (IC) for controlling LED string current. LED strings typically require higher voltages than the IC to control the LED string current. For example, in a typical application an LED control IC might run from 12 volts, while the LED string might run from 40 volts. Linear circuits can be used to generate the proper voltage for the IC, such as a simple or active shunt circuit or a shunt with an external NMOS. However, these circuits can add costs, die area and components.